Still enjoying this tank but as for the Eheim, I read above about it having bubbles? My 2215 will randomly throw out a few seconds of bubbles with a grinding noise. It is not very old; roughly 6 months; and I have never broke or replaced anything.
So to sum up fixing the problem the way you did would be to take out the magnet and gently replace it into the plastic housing with a final lubrication of everything with Vaseline?
my issue was a little different. I had filter issues because I had initially snapped the ceramic rod that the impeller spins on when I was cleaning the filter. I ended up replacing the impeller unit all together because my old impeller had more of an oval hole that the ceramic rod had created because of my neglect.
I agree with trigger below. sounds like you just have air in yours as well. If you tilt your filter the bubbles should dislodge. make sure all of your connections are secure. You can always check your impeller, but if your filter is only 6 months old, probably best not to mess with it unless the noise coming from it is bothersome.
That just means you have some air trapped in there. My 2215 does the same thing until I pick it up while it's running and shake it around until I get all of the air out. Usually only happens after I open it for some cleaning, or after I do a water change and the water level drops below the intake screen